The invention relates to a method for growing at least one silicon carbide (SiC) single crystal by sublimation of a SiC source material.
The sublimation of industrial-grade SiC in powder form and growing the SiC in a gas phase on a monocrystalline SiC seed crystal is a method for the production of a SiC single crystal known from German Patent DE 32 30 727 C2. The gas mixture that is formed during the sublimation primarily contains the components Si, Si2C, SiC2 and SiC. The gas mixture is also referred to below as xe2x80x9cSiC in the gas phasexe2x80x9d. The source material used is usually industrial-grade silicon carbide in granular form and of high purity, the silicon carbide preferably having a grain size of approximately 200 to 300 xcexcm. The SiC source material is produced prior to the growing process of the SiC single crystal and is introduced as a prepared source material into a growing chamber.
The article titled xe2x80x9cSiC-Bulk Growth By Physical-Vapor Transport And Its Global Modelingxe2x80x9d, by D. Hofmann et al., Journal of Crystal Growth, Vol. 174, 1979, pages 669 to 674 describes a method for the synthesis of a SiC powder which is used as a source material from elemental silicon (Si) granules and carbon (C) powder.
Another method for producing the source material in the form of SiC powder is known from the article titled xe2x80x9cImprovement In The Growth Rate Of Cubic Silicon Carbide Bulk Single Crystals Grown By The Sublimation Methodxe2x80x9d, by H. N. Jayatirtha et al., Journal of Crystal Growth, Vol. 174, 1997, pages 662 to 668. In this case, high-purity silicon and high-purity carbon react with one another for a total of three hours at 1800xc2x0 C. Then, the powder is removed from the reaction furnace and is subjected to further treatment steps. In particular, a three hour oxidation is carried out at 1200xc2x0 C., in order to remove excess carbon. Then, an etching step is carried out, in order to eliminate traces of SiO2 that have formed as a result of the oxidation.
A common feature of all the above-mentioned methods for the production of the source material in the form of the SiC powder is that the production of the SiC powder and the actual growing of the SiC single crystal represent individual processes that are carried out independently of one another.
By contrast, Published Japanese Patent Application JP 6-316 499 A describes a method for the production of an SiC single crystal which in one first process step also includes the formation of the SiC source material from silicon and carbon as an integral part of the growing process. The growing operation in the narrower sense takes place as a result of the sublimation of the SiC source material produced in the previous process step and as a result of deposition of the SiC in the gas phase on an SiC seed crystal. The starting materials used for the production of the SiC source material are carbon in the form of a graphite block or carbon (C) powder with a grain diameter of below 10 xcexcm and granular silicon with a grain diameter of between 2 and 5 mm. The grain diameter of the silicon is given as a parameter that is not critical and, if appropriate, may also assume values outside the diameter range stated. However, for the C powder employed it is expressly stated that if the grain size is over 10 xcexcm only the surface of the C powder reacts with silicon, and therefore the SiC source material formed is not suitable.
In a second embodiment, in which a graphite block of porous graphite with a relative density of 0.5 g/cm3 is used instead of the C powder, the production process of the SiC source material and the growing process of the SiC single crystal do not take place within a single process sequence. The growing chamber is opened again after the SiC source material has been produced, in order for the SiC seed crystal to be introduced. Only then is the actual growing commenced. The relative density of the porous graphite used may also adopt values other than 0.5 g/cm3. The upper limit specified is a relative density of 1.0 g/cm3, since at higher levels SiC source material is no longer formed in sufficient quantities.
The process conditions during the production of the SiC source material involve a temperature in the range between 1150xc2x0 C. and 1800xc2x0 C. and a pressure of less than 200 Torr, corresponding to approximately 266 mbar.
When using C powder with a grain diameter of less than 10 xcexcm, the reaction taking place during the production of the SiC source material can very easily lead to fluidization of particles, in particular of the C powder. As a result, deposits are formed on the SiC seed crystal, which has an adverse effect on the quality of the growing which is then to be carried out.
It is accordingly an object of the invention to provide a method for growing SiC single crystals that overcomes the above-mentioned disadvantages of the prior art methods of this general type, which avoids the fluidization of the C powder which occurs in the prior art and therefore leads overall to an improved quality of the SiC single crystal which is grown.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for growing at least one silicon carbide (SiC) single crystal by sublimation of an SiC source material. The method includes introducing silicon (Si), carbon (C) and an SiC seed crystal into a growing chamber, the carbon being a C powder with a mean diameter of powder grains of greater than 10 xcexcm. Producing the SiC source material from the silicon and the carbon in a synthesis step carried out before a growing step; and carrying out the growing step by growing the SiC single crystal immediately after the synthesis step.
The invention is based on the recognition that, unlike in the customary procedure used in the specialist field, which is defined by the technical teaching disclosed by Published Japanese Patent Application JP 6-316 499 A, the use of C powder with a mean grain diameter of greater than 10 xcexcm is still possible and even advantageous. On account of the larger geometry and the consequently higher weight, this measure avoids the undesirable fluidization of the C powder during the production of the SiC source material. Therefore, during this process step there is no undesirable deposition on the SiC seed crystal, which is already in the growing chamber while the SiC source material is being produced. The recognition that C powder even with a mean grain diameter of greater than 10 xcexcm reacts with the silicon so well that the result is a SiC source material of high quality which is eminently suitable for the subsequent growing of the SiC single crystal is particularly surprising.
The carbon used may be both synthetic and natural carbon.
In a first preferred configuration, the C powder has a mean grain diameter of greater than 20 xcexcm, and in particular of greater than 30 xcexcm. The larger the mean grain diameter, the more successfully the undesirable fluidization of the C powder is suppressed.
An embodiment in which the C powder is selected to have a grain size which is less than 200 xcexcm and preferably less than 80 xcexcm is also advantageous. If the grain size is below the upper limits given above, all the carbon inside the powder grain still reacts with the silicon. By contrast, if a C powder with a larger mean grain diameter is provided, the carbon actually only reacts with the silicon at the surface of the powder grain. As a result, there is an undesirable excess quantity of unbonded carbon and silicon atoms in the SiC source material.
Another advantageous embodiment of the method involves a type of carbon in which the reaction with the silicon to form the SiC source material takes place endothermally or at least without significant amounts of energy being released being used for the carbon in the C powder. This is because undesirable fluidization of particles, in this case of both carbon and silicon particles, may also be caused by an exothermic reaction between the silicon and the carbon. The chemical energy released in this case leads to the undesirable fluidization with the adverse effects described above. With the preferred types of carbon mentioned, the reaction with the silicon takes place in particular without volumetric expansion. Thermoanalytical investigation of the reaction reveals that energy is absorbed or energy is released only to such a slight extent that it is too low to fluidize particles.
With a view to achieving a reaction between the C powder and the silicon which proceeds as far as possible without any energy being released, a variant in which a type of carbon whose powder grains are composed of a multiplicity of individual crystallites is particularly advantageously used. In this case, one powder grain preferably contains at least 10 to 20 crystallites. The polycrystalline structure of the individual grains then makes a decisive contribution to an overall reaction that proceeds substantially without energy being released. During heating, the silicon melts first. Prior to this, there is virtually no reaction between the two elements. The melting is endothermic. When melting commences, the reaction of the SiC synthesis, which in itself is exothermic, also starts. On account of the polycrystalline structure of the C powder grains, the synthesis reaction proceeds not only at the grain surface but also in the interior of the grains. Since the endothermic melting and the exothermic synthesis reaction take place substantially in parallel, the energy balance is practically neutral. In the case of a type of carbon whose powder grains contain only a few crystallites, by contrast, the two subreactions proceed successively. The advantageous energy compensation then cannot occur, and the energy that is released during the synthesis leads to undesirable fluidization of particles.
An embodiment in which the silicon is selected to be in the form of a silicon (Si) powder is also preferred. In this case, the maximum mean grain diameter of the Si powder is one millimeter. The mean grain diameter is preferably in a range between 100 xcexcm and 400 xcexcm. Compared to the grain size of the C powder, the mean grain diameter of the Si powder is a much less critical parameter. The reaction between silicon and carbon to form the SiC source material preferably takes place at temperatures of over 1400xc2x0 C. Since at these temperatures silicon is no longer in the powder form which was originally introduced, but rather is already in molten form, it is possible to select a significantly larger mean grain diameter for the Si powder than for the C powder.
The silicon, which has become liquid as a result of the melting operation, impregnates the carbon, so that the mixing between the silicon and carbon is determined almost exclusively by the mean grain diameter of the C powder. The upper limit for the mean grain diameter of the Si powder is determined by the fact that if the mean grain diameters are too large, the melting operation takes too long and therefore the carbon cannot optimally be impregnated by molten silicon.
To achieve mixing which is as homogeneous as possible for the reaction between the silicon and carbon, according to an advantageous configuration the Si powder and the C powder are intensively mixed together and then introduced into the growing chamber as a mixture which is as homogeneous as possible. As a result of the intensive mixing, the reaction yield during the production of the SiC source material is improved.
In a preferred variant embodiment, the growing chamber, in preparation for the production process of the SiC source material, is evacuated for several hours at a temperature of between 1000xc2x0 C. and 1300xc2x0 C., in particular 1200xc2x0 C., until a pressure of less than or equal to 10xe2x88x924 mbar is reached. After the evacuation, the growing chamber is filled with an inert gas, in particular with argon (Ar), helium (He) or hydrogen (H2). An inert gas pressure in a range between 100 and 1000 mbar is set.
A variant embodiment in which the silicon and the carbon are heated to a synthesis temperature of at least 1200xc2x0 C. and at most 1900xc2x0 C. is also advantageous. A synthesis temperature in the range between 1400 and 1600xc2x0 C. is particularly advantageous, since in this temperature range on the one hand the silicon has already melted and impregnated the carbon and on the other hand the reaction to form the SiC source material takes place with a good yield. The synthesis temperature is maintained for a period of at most three hours, in particular of only at most one hour, so that the silicon which is introduced into the growing chamber and the carbon which is also introduced react with one another as completely as possible and form the desired Sic source material.
A particular advantage of the method resides in no further cooling of the growing chamber and, if appropriate, removal of the SiC source material from the growing chamber after the SiC source material has been produced, since these steps could introduce undesirable impurities into the SiC source material. Therefore, an advantageous variant embodiment involves the growing chamber, starting from the conditions for the production of the SiC source material, being brought to the growing conditions for the SiC single crystal. This results in a rise in the temperature of the SiC source material from the synthesis temperature to a temperature of between 2100xc2x0 C. and 2500xc2x0 C. Moreover, at the same time the overall pressure that prevails during the production of the SiC source material is also reduced to at most 30 mbar. Under these conditions, SiC is sublimed out of the SiC source material and then precipitates out of the gas phase on the SiC seed crystal. To ensure corresponding material transport of the SiC in the gas phase, a temperature gradient is established between the SiC source material and the SiC seed crystal or the SiC single crystal that is being formed.
By suitably selected orientations of the SiC seed crystal, it is possible to grow a SiC single crystal of a specific polytype. Therefore, in advantageous embodiments there is provision for the SiC seed crystal to be disposed in the growing chamber in such a way that the SiC single crystal grows on a C side or an Si side of the SiC seed crystal in the direction of a polar axis. The growth on the C side provides a 4H-SiC single crystal, whereas the growth on an Si side provides a 6H-SiC single crystal.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for growing SIC single crystals, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.